During 5.8, we discovered that an update to the 5.8 kernel (2.6.18-308.el5) will break the VLAN setup due to a bug in the updated tg3 kernel module
More informations can be read on the upstream bugzilla tool : https://bugzilla.redhat.com/show_bug.cgi?id=798939 [^]

Just to add that the current workaround (except staying with the pre-5.8 kernel of course) is to install the kmod-tg3-rhel5u7 from the CentOS updates repository.
Even if it's marked as 5u7, it will work on 5.8 and fixes the 802.1q issue introduced in the tg3 module from the 5.8 kernel